It is known that bulk material such as granular or particulate material can be conveyed through pipes or conduits by means of carrier gas such as atmospheric air. However, for reasons of economy it is desirable to maximise the amount of material conveyed for a given amount of carrier gas, as this has the advantage that small gas compressors and conduits and small separators can be used in the conveying equipment. In addition, low speeds of material and gas flow reduce wear on the walls of the conduit, while there is also less likelihood of damage to the particles of material being conveyed. However, if the speed of flow in the conduit falls below a minimum speed, material can drop out of the gas flow and become deposited in the conduit, and this can readily pile up and form a blockage which can block the conduit entirely.
It is also known that a by-pass conduit may be arranged in parallel with the conveyor conduit, the by-pass conduit being in communication with the conveyor conduit at positions spaced along the conduits so that clean gas can be injected into the conveyor conduit from the by-pass conduit, so as thereby to break up a blockage of material. A method and apparatus operating on this principle are disclosed in Krambrock U.S. Patent No. 3,929,379 issued on Dec. 30, 1975 and assigned to the assignees of the present application.
Accordingly, U.S. Pat. No. 3,929,379 discloses for the pneumatic conveying of bulk material by means of a conveyor conduit and a by-pass conduit which has communications with the conveyor conduit at a plurality of spaced positions. The pressure distribution in the bypass conduit is substantially adapted to the pressure distribution which occurs in normal trouble-free conveying operation in the conveyor conduit. When there occurs in the conveyor conduit an accumulation of material which impedes flow in the conveyor conduit, there is accordingly a pressure drop in the conveyor conduit downstream of the accumulation. Such a pressure drop causes a pressure difference in excess of a preselected threshold value between the two conduits, and the pressure difference causes the transfer of clean gas from the by-pass conduit into the conveyor conduit to loosen the accumulation of material. At the same time the flow of clean gas through the by-pass conduit is blocked at the communication position at which gas is being transferred from the by-pass conduit into the conveyor conduit. In an apparatus which is adapted for carrying out the method as outlined above, the by-pass and conveyor conduits are in communication with each other by way of a plurality of connecting means spaced along the lengths of the conduit, and non-return valves are disposed in each of said connecting means, such that the respective valve opens for the transfer of gas from the by-pass to the conveyor conduit when the pressure in the by-pass conduit is higher than the pressure in the conveyor conduit by the predetermined threshold amount. Associated with each non-return valve is a shut-off valve operable to close the by-pass conduit when the respective non-return valve is opened for the transfer of gas as aforesaid.
The basic concept of U.S. Pat. No. 3,929,279 is therefore based on the feature that the pressure difference between the by-pass conduit and the conveyor conduit, which occurs when there is a concentration or blockage of material, is utilised for operating the flow control valves thereby to divert a flow of clean gas from the by-pass conduit into the conveyor conduit and to shut off the flow of gas further along the bypass conduit. It might be said therefore that each of the flow control valves not only acts as a valve for controlling the flow of gas, but also operates as a pressure measuring member responsive to the respective pressures prevailing in the conduits. This duality of functional requirements can result in the valves necessarily being of an expensive construction, while also exhibiting a certain liability to breakdown.